The invention relates generally to apparatus for generating suitable output signals in accordance with signals generated by photovoltaic devices. More specifically, the invention concerns amplifiers for processing signals generated by photovoltaic detectors and supplying the processed signals as outputs to suitable utilization devices.
Focal plane arrays of infrared detecting photovoltaic devices, such as diodes, are used for imaging objects which emit infrared radiation. For example, suitable infrared emitting objects would be relatively warm and could be targets to be acquired by heat seeking weapon systems. Each photovoltaic detector in such an array serves as a picture element or "pixel" and read-out apparatus associated with each pixel is required to integrate the current generated by photons impinging on the infrared photovoltaic device. Integration of the acquired optical signal is desired to minimize target signal noise and to enable acquisition of randomly arriving photons at the detector device.
To convert the integrated photovoltaic detector current to an output voltage for use by a processing device, it has been known to couple the detector output to a transimpedance amplifier which basically serves to generate an output voltage determined by an input current supplied from a photovoltaic device, the value of the output voltage being related to the transimpedance of the read-out amplifying device. While operationally satisfactory, such read-out devices, however, tended to induce deblasting of the detector, which leads to increased noise characteristics. Increased noise, in turn, requires use of relatively high bias voltages for the detector.
One known approach to providing suitable read-out amplifiers for photovoltaic detectors utilizes a series connected autozero capacitor connected between the photovoltaic detector and an integrating amplifier field effect transistor gate. When the known amplifier is reset between read-out integration intervals, noise on the autozero capacitor is held on the photovoltaic detector for the duration of each integration read-out, thereby causing a noise current through the dynamic resistance of the photovoltaic device. This noise current is integrated causing an output noise much larger than the noise voltage initially generated at the autozero capacitor. There is, therefore, seen to be a need for an improved read-out amplifier for a photovoltaic detector which will exhibit improved noise characteristics and minimal debiasing, thereby enabling the photovoltaic detector to operate at relatively small bias voltage levels.